This invention relates to an electronic musical instrument of a waveform memory reading type, and more particularly to a polyphonic electronic musical instrument having a plurality of tone production channels.
A polyphonic electronic musical instrument, particularly an electronic musical instrument of the digital processing type is provided with a plurality of tone production channels and key information of a depressed key is assigned to one of the tone production channels by a key assigner for producing a musical tone by the assigned tone production channel in accordance with the key information. As a method of generating the musical tone by the designated tone production channel, there is known a method of reading out a musical tone waveform from a waveform memory device. In one example, a plurality of tone production channels are connected in parallel on a non time division basis and independent waveform memory devices are provided for respective channels. In this system, however, it is necessary to provide elements required for forming a musical tone for every channel. Especially, the waveform memory device provided for each channel has a relatively complicated construction so that provision of such a waveform memory device for each channel not only increases the size of the circuit construction of the electronic musical instrument but also increases the cost of manufacture. Moreover, in order to enable switching between waveforms (switching between tone colors) it is necessary to provide a plurality of waveform memory devices for each channel which makes it difficult to simply switch waveforms (tone colors) without increasing the manufacturing cost and circuit construction size.
According to one approach to these problems, a plurality of tone production channels are formed on a time division basis so as to use commonly one waveform memory device for respective channels on the time division basis. This system is disclosed, for example, in U.S. Pat. No. 3,882,751 dated May 13, 1975. When compared with the aforementioned system, this system is advantageous in that it can save the manufacturing cost and the number of component elements but as the time division frequency is set independently of the musical tone frequency, the musical tone waveform read out on the time division basis contains non-harmonic tones thus distorting the tone produced. Generally, since the time division frequency is in a range higher than audible frequencies it does not act as a non-harmonic tone component but as is apparent from sampling theorem, where a musical tone wave of an especially high frequency is read out on the time division basis an aliasing (frequency reflection) noise which is not harmonic with the musical tone frequency might be generated. In addition to the problem of the aliasing noise, when the time division time slots change instantaneously for the respective channels, the musical tone waveform sampling amplitudes interfere with each other between adjacent channels at every time slot, thus distorting the waveform and making the tone unclear.